The hard-living “Entourage” actor claims that he’s too sick to show up for his Broadway gig – and that a mysterious “mercury” ailment has forced him to abandon the stage.

Piven missed Tuesday night’s per formance – as well as yesterday’s matinee and evening performances – of “Speed-the-Plow,” much to the anger of ticket holders.

Piven, who flew to Los Angeles last night, is under con tract to perform throughout the show’s run, which began in October and is slated to end Feb. 22.

Over the past few weeks, the star had complained of “exhaustion” and “being tired,” sources told The Post.

The popular play’s investors aren’t buying it – and are discussing calling in their own doctor in to get a “second opinion,” the sources said.

Daily Variety reported that Piven said he was suffering from a “high level of mercury,” leading Pulitzer Prize-winning playwright David Mamet, who wrote the showbiz satire, to remark tartly, “My understanding is that he is leaving show business to pursue a career as a thermometer.”

Piven’s reps did not elaborate on what was ailing the Emmy Award-winning actor.

Piven, 43, is famous for playing fast-talking, party-animal agent Ari Gold on the HBO hit “Entourage.”

Sources said that Piven hasn’t been too sick to party after performances – hanging out at bars into the wee hours, despite his grueling schedule of eight performances a week.

Witnesses have told The Post that Piven is a regular at the Lower East Side bar The Eldridge.

“He lives his life very much like the way his character on ‘Entourage’ does,” said one source.

Investors are worried that without Piven – one of the play’s main attractions, who had earned up to $15,000 a week – they might not be able to recoup their $3.5 million outlay, on which they figured they’d break even by next week.

At yesterday’s matinee, more than 300 ticketholders demanded a refund when they heard Piven was a no-show.

The box office has already handed out roughly $20,000 worth of refunds because of his absence.

Piven previously had been showing up late to performances each night, barely making it to the Barrymore Theatre on West 47th Street before the curtain went up. Under union rules, he is required to sign in by 7:30 for an 8 p.m. curtain.